---------Direct observation of the miscibility improving effect of ultra-small polymeric nanoparticles (radius ~4 nm) in model systems of soft nanocomposites is reported. We have found thermodynamically arrested phase separation in classical poly(styrene) (PS) / poly(vinyl methyl ether) blends when PS linear-chains were totally replaced by ultra-small, single-chain PS nanoparticles as determined by thermo-optical microscopy measurements. Partial arrested phase-splitting on heating was observed when only some of the PS chains were replaced by unimolecular PS nanoparticles leading to a significant increase of the lower critical solution temperature (LCST) of the system (up to 40 ºC at 15 vol. % nanoparticle content). Atomic -2 -force microscopy and rheological experiments support these findings. Thermodynamic arrest of the phase separation process induced by replacement of linear-polymer-chains by unimolecular-polymer-nanoparticles could have significant implications for industrial applications requiring soft nanocomposite materials with excellent nanoparticle dispersion in a broad temperature range. IntroductionNanoparticles are currently ubiquitous in the nanotechnology arena showing a great impact in organic photovoltaics, [1] nanophotonics, [2] catalysis, [3] drug delivery, [4] and nanomedicine, [5] among other several fields. [6] A strong effort has been devoted in recent years to the efficient and shape-controlled synthesis of metallic, metal oxide and semiconducting (quantum dot) hard nanoparticles (HNPs). It is now well-established that HNP surface properties are mainly determined by chemical composition, surface chemistry (hydrophobic/hydrophilic balance), shape, size, porosity, roughness, and compositional heterogeneity. [7] Even when the progress in this field has been astonishing, highly-dense HNPs are sometimes far from ideal when employed in conjunction with soft matter (e.g. biomacromolecules). Compared to classical HNPs, the multi-gram synthesis of well-defined unimolecular polymeric nanoparticles in the sub-20 nm size range has been elusive until recently. The pioneering work of Hawker´s group[8] opened the way to new and highly efficient synthetic routes to monodisperse, ultrasmall (radius < 5 nm) soft nanoparticles (SNPs). [9][10][11][12][13][14][15][16] The rich phase behavior of nanocomposites containing HNPs has been investigated by combining computer simulations, [17,18] theory [19][20][21][22] and experiment. [23,24] Conversely, the miscibility (i.e.homogeneity at the nanoscale) behavior of soft nanocomposites consisting on unimolecular SNPs dispersed in a conventional polymer matrix remains largely unexplored, even if mimicking Nature´s soft nanocomposites is currently of great interest. Most thermodynamic (i.e. homogeneity) and rheological (i.e. dynamics) data correspond to simple blends of -3 -intramolecular cross-linked polystyrene (PS)-nanoparticles dispersed in a matrix of PS linearchains, displaying interesting non-Einstein viscosity behaviour. [25,26] Also computer simulations [27...

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